Micromachined Nanoparticulate Ceramic Gas Sensor Array on Mems Substrates
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MICROMACHINED NANOPARTICULATE CERAMIC GAS SENSOR ARRAY ON MEMS SUBSTRATES
Martin Heule and Ludwig J. Gauckler Nonmetallic Inorganic Materials, ETH Zurich Sonneggstrasse 5, 8092 Zurich, Switzerland.
ABSTRACT In most MEMS applications, dust and particles are avoided with considerable endeavor. However, for many applications such as gas sensors, powders of functional ceramics would often provide better performance than corresponding thin film layers. There are abundant ways to synthesize powders with well defined chemical composition, phase and size distribution, whereas the processing parameters for thin-film preparation often are limited. Specifically where the functionality is based on a chemical reaction on surfaces, nanoscaled powders with a high specific surface area have proven useful. This is the case for tin oxide gas sensors that exhibit a drop in electrical resistivity induced by the combustion of the analyte gas. Soft lithography was used for the fabrication of mesoscaled powder-based ceramic structures. In this paper, we present the integration of small tin oxide microstructures with an effective gas-sensing area of 10 by 30 µm2 on a micro-hotplate substrate. Such a substrate can heat the ceramic sensor to operating temperatures quickly with low power consumption. A whole array of sensors can be integrated on one micro-hotplate. Processing steps to prepare the sensor array on the micro hot plate are presented and discussed concerning processing sequence, sensitivity towards 1000 to 1500 ppm hydrogen and power consumption. Additionally, effects of grain growth due to on-chip annealing of the ceramic nanostructure were observed.
INTRODUCTION Micro fabricated semiconducting gas sensors consist of a sensitive ceramic layer that is typically supported on a thin freestanding membrane that can be heated to temperatures up to 500°C 1-3. The most significant advantages of miniaturized designs besides their small dimensions are the low power consumption in the range of mW and quick response times. Among the materials often used are tin oxide (SnO2), tungsten oxide, titanium oxide and many others 4. The range of detectable analyte gases include reducible gases like hydrogen, carbon monoxide and ethanol. Therefore, target applications are warning sensors in securityrelevant situations as well as room climate monitoring. As far as the gas sensing process is understood today, the topmost layer of oxygen is removed by a catalytic combustion of the analyte gas on the semiconductor surface. This catalytic processes enable the sensors to detect very sensitively. However, selectivity towards a specific gaseous species in a gas mixture is often the limiting obstacle that prevents the application of these resistive type gas sensors. To a certain extent, the selectivity may be tailored by adding dopants to the ceramics. One possible solution is the use of an array of several differently reactive sensors simultaneously. The combination of their responses in multivariate analysis or pattern recognition techniques allows for un
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